[InstSimplify][IR] Handle trapping constant aggregate (PR49839)

Unfortunately, it's not just constant expressions that can trap,
we might also have a trapping constant expression nested inside
a constant aggregate.

Perform the check during phi folding on Constant rather than
ConstantExpr, and extend the Constant::mayTrap() implementation
to also recursive into ConstantAggregates, not just ConstantExprs.

Fixes https://github.com/llvm/llvm-project/issues/49839.